In the manufacture and assembly of electric and electronic units, the electronic components are commonly mounted on preformed circuit boards. The most common technique used in making circuit boards are the so called "printed circuit" techniques where conductor patterns are chemically etched or deposited on the surface of a substrate. Recently, however, techniques have been developed for making circuit boards by scribing or writing insulated wires on to the surface of the board. The advantages of the scribed technique are that the system can be computer controlled (eliminating the need for printing art work) and high density patterns can be created (due to virtually unlimited conductor crossovers).
The basic scribed wire technique is described in Burr Pat. No. 3,674,914. The insulated wire, as it is written or scribed, is fixed or tacked by a heat sensitive adhesive surface on the substrate by means of a scribing or tacking head. The head guides the wire and heats or energizes the sensitive surface as the wire is brought into engagement. The preferred technique for tacking the scribed wire is through the use of ultrasonic energy. The wire passes beneath a grooved stylus which is used to position the wire. Ultrasonic energy is applied to the stylus to activate the adhesive layer beneath the wire and to push the wire into the adhesive layer. After the tacked or scribed wire pattern is complete on the board surface, the pattern is fixed by coating the board with an encapsulating layer. Thereafter, holes are drilled in the board at the conductor run terminations and the holes then plated to provide for surface connections to electronic components.
The Burr '914 patent discloses a system suitable for making discrete wired interconnection boards for the dual-in-line integrated circuits which have been in common use for many years. The standard pin spacing for these integrated circuits is 100 mil. Interconnection boards for this pin spacing are made using No. 34 AWG wire having a 0.0063 inch (6.3 mil) diameter. Plated holes for providing surface connections and for accommodating component lead wires are on the order of 46 mil. With these parameters conductor displacement errors as great as 16 mil can be tolerated without faults resulting from missing the conductor or connecting to the wrong conductor during the drilling operations.
Because of the small wire size, the energy required, and the available space within the scribing head, the ultrasonic stylus is preferably energized by magnetostriction and is relatively long and narrow. Since the nodal points where the stylus can be rigidly held are at least several inches from the working tip of the stylus, additional positioning stability is needed near the working tip. In the past, additional positioning stability was provided by means of a Teflon bushing with a 1 mil clearance to permit longitudinal vibration of the stylus. With this arrangement the ultrasonic stylus could position the scribed wires with sufficient accuracy for making the dual-in-line circuit boards with the 100 mil pin spacing.